This invention relates to a drive control circuit for d.c. motor, in particular, to a control circuit for driving a d.c. motor having its associated tachogenerator, in synchronism with a standard frequency.
Various schemes are proposed to rotate a d.c. motor at a constant speed, a typical one being a phase control scheme. In this scheme, the rotation of the motor is controlled so as to maintain the frequency from a tachogenerator which is mechanically connected with the motor in coincidence with a standard frequency from a standard frequency generator, as schematically shown in FIG. 1. Specifically, a phase discriminator PD has an input terminal 10 which is adapted to receive a standard frequency, and another input terminal 20 which is adapted to receive the frequency from a tachogenerator G associated with a d.c. motor M. Any difference between the input frequencies is detected to derive a voltage, which is amplified by a d.c. power amplifier A and applied to the motor M for controlling the speed thereof. In FIG. 1, the letter L designates a suitable load on the motor.
The phase discriminator PD can be constructed in various forms, and a motor control circuit is shown in FIG. 2 in which the phase discriminator comprises a set-reset RS flipflop circuit. The RS flipflop circuit FF has one input terminal 10 to which a standard frequency is applied and another input terminal 20 to which the frequency signal from the tachogenerator G is applied through a waveform shaping circuit WS. As is well recognized, when a low level (L) pulse is applied to the input terminal 10, an output H of a high level is produced and maintained at its output terminal 30 even if low level pulses subsequently applied to the input terminal 10. However, when a low level pulse is applied to the input terminal 20, the output of the flipflop circuit is inverted to a low level L, and such output level is maintained even if low level are subsequently applied to the input terminal 20. In other words, the output of the RS flipflop circuit FF changes between levels H and L if the frequency signal from the tachogenerator G appears within the period of the standard frequency. When the frequency signal from the tachogenerator G gains in phase with respect to the standard frequency signal, the pulse width at the output 30 of the flipflop circuit FF will be reduced while it will be increased when the phase of the frequency signal from the tachogenerator G becomes lagging. If the frequency signal from the tachogenerator G appears at the middle point of the pulse interval of the standard frequency signal, the output of the flipflop circuit FF will be at its high and low levels for an equal interval which is just one-half the pulse interval of the standard frequency signal. FIG. 3 shows various conditions of the control circuit shown in FIG. 2. In FIG. 3(a), the frequency signal from the tachogenerator is slightly lagging in phase with respect to the standard frequency signal, so that the output of the RS flipflop circuit FF will have a reduced pulse width. As a consequence, an average output voltage E.sub.0 of an integrator INT (see FIG. 29 which integrates such pulses is low. In FIG. 3(b), the phase lag of the tachogenerator signal with respect to the standard frequency signal is increased, whereby the output pulse of the RS flipflop circuit FF will have an increased width, thus increasing the average output voltage E.sub.0. In this manner, the control circuit provides a feedback in a manner such that the operation is stabilized at a phase which is determined by the overall system. Specifically, the phase control will be effective in a range from 0 to -2.pi. as represented in terms of a phase difference between the standard and tachogenerator frequency signals, and a stable motor rotation is possible through the phase control within such range.
FIG. 3(c) illustrates a starting operation of the motor, illustrating the phase control during the time the motor rotation gradually increases from its stop condition to a number of rotation at which the frequency signal from the tachogenerator G becomes equal to the standard frequency. However, in practice, the output of the RS flipflop undergoes a repetition of H and L signals having varying pulse widths until the tachogenerator frequency reaches the standard frequency, as illustrated in FIGS. 3(d) and (e). This involves the possibility that the motor rotation may reach a condition of equillibrium at which the tachogenerator frequency is below the standard frequency, depending on the magnitude of the load on the motor and the moment of inertia of the system. The motor rotation may be fixed to a given number of revolutions, or may increase and decrease periodically within a given range of numbers of revolutions. As will be appreciated, the motor rotation may be reduced to one-half its initial value by changing the standard frequency to one-half its initial value or by subjecting the tachogenerator frequency signal to a full wave rectification to provide a doubled frequency. In this instance, however, since the motor rotation gradually reaches the final number of revolutions which is one-half the initial value after repeating H and L signals of varying pulse widths, as shown in FIGS. 3(f) and (g), again there is the possibility that the motor rotation may not reach a desired number of revolutions by becoming balanced at a different number of revolutions which depends on the magnitude of the load and the moment of inertia. Additionally, with the circuit shown in FIG. 2, it takes a considerable length of time to reach a desired number of revolutions since an increase or decrease in the number of revolutions involves a repetition of H and L signals.